Add Dependabot badge to readme (#414)

Author: fgreinacher

README.md

@@ -1,6 +1,6 @@
 [![NuGet](https://img.shields.io/nuget/v/System.IO.Abstractions.svg)](https://www.nuget.org/packages/System.IO.Abstractions)
-[![Windows build status](https://ci.appveyor.com/api/projects/status/em172apw1v5k70vq/branch/master?svg=true)](https://ci.appveyor.com/project/tathamoddie/system-io-abstractions/branch/master) on Windows
-[![Linux build status](https://travis-ci.org/System-IO-Abstractions/System.IO.Abstractions.svg?branch=master)](https://travis-ci.org/System-IO-Abstractions/System.IO.Abstractions) on Linux
+[![Windows build status](https://ci.appveyor.com/api/projects/status/em172apw1v5k70vq/branch/master?svg=true)](https://ci.appveyor.com/project/tathamoddie/system-io-abstractions/branch/master)
+[![Linux build status](https://travis-ci.org/System-IO-Abstractions/System.IO.Abstractions.svg?branch=master)](https://travis-ci.org/System-IO-Abstractions/System.IO.Abstractions) [![Dependabot Status](https://api.dependabot.com/badges/status?host=github&repo=System-IO-Abstractions/System.IO.Abstractions)](https://dependabot.com)
 
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